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答案揭晓：

A sea of

大量

例句：

The square was a sea of people.

广场上人山人海。

A sea of faces:

人山人海

例句：

There was a sea of faces on every scenic spot in China during Spring Festival.

春节期间，中国的每个景点都是人山人海。

A crowd of:

一大群；一群

例句：

A crowd of youths started smashing windows.

一群年轻人开始砸窗户。

Assignment（脑洞大开时间）

Lame duck

Song for you（片尾曲）

FrogMonster - Remember Our Summer